Spin Bikes: What is Better Light or Heavy Flywheels?

We’ve had a few questions lately asking whether it is better to have heaver or lighter flywheels on exercise bikes. It all started when Groom+Style reviewed the top 5 spin bikes, and gave top spot to the Keiser M3 plus (M3i and M3iX range of bikes).

The Keiser is a bike that Groom+Style absolutely love, and there are a lot of reasons why we gave it that top spot. The Keiser M3 range of bikes do however have one of the lightest flywheels of any spin bike at 8lbs.

The Keiser M3 Plus Indoor Cycle with its rear positioned 8lbs flywheel. Does it generate enough inertia to provide a real bike feel?

This has caused a few readers to question how Groom+Style can rate this bike so highly when there is quite a bit of older information “out there” mentioning that heavier flywheels are better.

The story goes that light flywheels do not create enough inertia which can cause some riders to develop an “up-down” or choppy pedal stroke – which in turn can lead to stress on joints and ultimately injury.

In this article Groom+Style will explain why this light flywheel myth is incorrect, at least when it is applied to a high end, well engineered spin bike like the Keiser.

What Is the Flywheel on a Spin Bike and What Does It Do?

First off, let’s get this out there in case you are a complete beginner, looking to get into spinning. The Flywheel is the big round disc that spins around underneath the bike. It can be found at the back or the front and is the most prominent feature on most spin bikes.

The flywheels job is to help to simulate the feel of an outdoor bike – it does this by storing energy which is then used to smooth out the cycling motion to promote a consistent cadence (the speed at which your legs spin as they pedal).

Specifically, on a fixed gear indoor spin bike, the amount of force you can exert changes during the pedaling motion. At the point when your pedals are horizontal you are exerting maximum force, but when the pedals are vertical (straight up and down) you are applying a lot less force and the wheels can start to slow. By having some momentum built up in the flywheel, you have less slowing of the pedal stroke and a more consistent pedaling action.

In addition, if you think of the initial feeling of resistance you experience when you start to ride a bike outside – that is the feeling the flywheel on an indoor bike is trying to replicate.

If there was no flywheel, or an ineffective flywheel on your spin bike, it would be very uncomfortable to ride, and could eventually lead to the rider developing an injury.

Flywheel Designs – The Different Options Available and What is the Best Design

For the flywheel to perform its function correctly it needs to store a certain amount of kinetic (moving) energy to accurately simulate the smooth pedal motion of a road bike.

A “good” flywheel will typically take some force to set it spinning (think of the force required to get an outdoor bike going); and will typically take some force to make it stop (think of how your outdoor bike will keep going for a while once you stop pedalling, unless you apply the breaks of course).

Kinetic energy is a function of mass and speed, flywheels can, therefore, come in all different shapes and sizes. Interestingly, the laws of physics tell us that large diameter and heavy flywheels store more energy than smaller diameter and lighter flywheels. However, flywheels that spin faster store much more energy than ones that spin slower. This is the key point that you need to remember when thinking about the Keiser flywheel.

Putting this another way…

The physics of flywheels tells us that if you take a heavy flywheel and double its weight it will store twice as much energy if you keep the speed the same.

However, if you take the same flywheel and spin it twice as fast, you will quadruple how much energy it stores. That is why top flywheel designers (think racing cars, and yes the Keiser spin bikes) use high-speed flywheels rather than heavy, massive ones.

Bike Designers Have Two Main Options When Designing Their Flywheels

Therefore, spin bike and flywheel designers can either use:

– heavier flywheels which spin at relatively slower speeds. For example, this is the option chosen by Schwinn etc.

The 37lbs flywheel on the Schwinn is positioned at the front of the bike.

– a lighter flywheel which spins faster. This is the option chose by Keiser. The Keiser flywheel is the fastest rotating flywheel of any indoor cycle bike in the world, it rotates 11 times for every single rotation of the actual pedals.

The Keiser flywheel is therefore designed to have more than enough inertia in the flywheel to ensure a smooth ride, while not being too aggressive.

In fact, from our experience, the Keiser spin bikes are the most comfortable and forgiving bike on the market for the ‘non-cyclist’ whilst giving excellent muscle recruitment feedback due to the constant resistance throughout the stroke itself. There is no stamping motion promoted because there is no ‘grabbing’ of the flywheel at any point during the pedaling rotation.

The Impact of Different Flywheel Weight Designs on the Pedal Stroke and Muscles Recruitment

Heavier flywheel weighted bikes can have a tendency to pull the legs through the full 360 degrees of the pedaling action putting unnecessary pressure on the knees and actually removing the need to recruit muscles i.e. you can get lazy during your spin class which defeats the purpose.

In contrast, the Keiser’s lighter flywheel design can help to promote hamstring and glute recruitment on the upstroke of the pedal stroke helping you to work harder.

Bad Pedalling Technique and Bike Setup

At the end of the day, it is possible to have “bad” pedaling technique with any spin bike, and whether you choose to buy a bike with a lighter or heavier flywheel it is probably more important for long-term health and injury prevention that you:

– ensure that you have setup the spin bike to fit you correctly. There are plenty of tutorials online about how to do this.

– that you stay focused throughout your spinning workout and focus on a smooth fluid pedal stroke. If you find your pedaling action getting out of control, back of a bit and refocus on your technique.

Flywheel Weight Design and Implications for the Rest of the Spin Bike

Interestingly, the weight at which you design the flywheel can also have design implications for the rest of the bike.

Bikes with heavy flywheels end up having to position the flywheel (as well as the braking system, gearing system and bearings) underneath the sweat zone of the rider, otherwise, the bike would be too heavy to move. As you can imagine putting these key components in the sweat zone is not the ideal design, as it increases the risk of wear and tear – which in turn means these bikes will require more maintenance.

With the Keiser’s lighter flywheel it can be positioned at the rear of the bike, away from the sweat zone, so the bike can still be moved easily.

Heavy vs. Lighter Flywheel Weight Conclusions

Based on this information you can see that not only is the Keiser’s lighter, faster spinning, rear positioned flywheel optimally designed, but it will have more than enough inertia to ensure a smooth and comfortable spin bike experience.

Although they are important, flywheels, however, are not the only factor you need to consider when buying a spin bike – you still need to focus on correct bike fit and setup, and your pedaling technique.

As a high-end spin bike is a big investment, if you are unsure about whether a bike with a heavy or lighter flywheel is best for you Groom+Style recommend that you contact your local cycling representative (of the brand bike you are interested in) and organize for test ride.

About The Author

Thanks for your comment, I have not been able to find the relative inertia comparison details between the M3i and the AC performance Plus, but during my investigations and discussions with Keiser and Schwinn I was able to conclude the following.

The inertia comparison between a Keiser m3i and a Schwinn bike at 80 RPM would be very similar because it is the objective of both bike manufacturers to achieve a smooth and realistic pedal rotation at a given RPM. The difference between the Keiser bike and the Schwinn bike is that because the Schwinn bike rotates it flywheel at a slower speed it will gain its inertia from mass rather than the way Keiser gains its inertia from the velocity in its flywheel (as we tried to explain in the article).

The main advantage of using velocity rather than mass to provide the required inertia is that at high speeds and low resistance the Keiser bike is always resisted but the Schwinn bike actually produce too much inertia and therefore the flywheel will drive the legs as opposed to the legs driving the flywheel.

Hope this makes sense.

Kind Regards,
Pierce

TerryJanuary 20, 2019

I am trying to compare the Keiser M3i to a Schwinn AC Performance Plus Carbon blue. The article needs some improvement to be meaningful to me. if the Keiser has a gear ration of 11 to 1 what is Schwinn’s gear ratio. And better yet when spinning at say a cadence of 80 what is the relative inertia comparison between the M3i and an AC performance Plus Carbon Blue?

Groom+StyleJune 19, 2018

Hi Jure,

Sorry for the delay, I have emailed you privately regarding the Kesiers flywheel specifications. Please also find the details below:

The diameter of the flywheel is 410mm at 3.6kg and the maximum power recorded on the console is 999 watts although using the app it is limited only by the speed and resistance, the rider can push i.e. theoretically unlimited.

Please let me know if you need anything else.

Kind Regards,
Pierce

JureJune 19, 2018

Hello,

thank you for offered link. Only one information is missing for my calculation. The diameter and mass of the pedal flywheel and mass of the 8lbs flywheel. I need to calculate the amount of stored kinetic energy in the system as certain frequency/cadence. Thank you again in advance.

Jure

Groom+StyleJune 18, 2018

Hi Jure,

Yes, please check out our write up linked below. Spoiler alert – we think it is a brilliant bike…

No problem, happy to help where we can. Let me see what we can dig up.

Kind Regards,
Pierce

JureMay 22, 2018

Hello,
I have maybe a strange question, but I am working a thesis on spinning bikes and I need some information. Of cource if there are any. What is the ratio between pedal spinning and flywheel spinning for Keiser M3i? 1:11 as stated in the article? What is the radius and mass of the Keiser M3i flywheel? What is the maximal power achieved with Keiser M3i? I need to do some mathematical analysis.

Thank you for the information!

Regards,
Jure

Groom+StyleMarch 11, 2018

Thanks Doug,

We did enjoy writing this article… 🙂

Cheers Pierce

DougMarch 10, 2018

Great article!,, I have been confused about videos I have seen indicating heavy perimeter
Weighted flywheels are safer etc. The laws of physics don’t lie, and your article nails it!

Groom+StyleJanuary 9, 2018

Hi Jon,

Thanks for taking the time to comment…we will see if we can find a contact at Keiser to pass on the feedback.

Kind Regards,
Pierce

Jon NorrisJanuary 8, 2018

Keiser really does make a great spin bike. This design has been around for years and is still the best spin bike IMHO. Recently, I tried the a copy of their bike by another manufacturer, but my ankles were hitting the pedal arms during class. Yes, those copy bikes have a fancy headlight, but the engineers who copied Keiser’s bike must have never taken a spin class.
The only improvement that I would like to see Keiser consider is to have their display not reset so fast. I have had to leave class to refill my water bottle, and found that the bike data reset back to zero. Also, when you stop the pedals and get average watts, please keep cycling the calories too.

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